feat(server): task 6.1/6.3 — dedicated server export preset and a real CLI surface

6.1: "Linux Dedicated Server" preset (dedicated_server=true,
custom_features="dedicated_server") mirroring the existing training
preset, plus run/main_scene.dedicated_server so the server binary reaches
its own entry point with no flag. Builds: an 85MB Linux x86_64 binary,
gitignored like the training one.

6.3: scripts/server_config.gd declares every server flag once - name,
type, default, section, help - and one parser turns that into parsing,
type checking, range validation, config-file backing and --help. The
flags had grown to ~30 across server_boot.gd and networked_match.gd, each
parsed inline with begins_with, none documented, and an unrecognised flag
was SILENTLY IGNORED: --max-clientss=8 ran a server on the default cap
and said nothing. Unknown flags, missing values, wrong types, duplicates
and out-of-range values are now hard errors, reported all at once.

Precedence is command line > config file > default. server_boot.gd parses
strictly because it owns the whole command line; networked_match.gd reads
the same declaration leniently because it is one consumer of an argv the
smoke harnesses also fill with --role= and --drive-seconds=. Nothing is
lost - every server flag is declared, so the strict pass already caught
any typo before the match scene re-reads its own.

13 unit tests covering the precedence order, the typo rejection that
motivated this, --no-<bool> not double-listing in --help, and --help
documenting every flag asserted against the declaration rather than a
hand-kept list. Verified end to end: --help prints, a typo'd flag refuses
to start, and the plain/replay-log/late-joiner smoke scenarios still pass.

Game/project.godot

@@ -21,6 +21,10 @@ run/main_scene="uid://bcq14356s3e2i"
 config/features=PackedStringArray("4.7", "Forward Plus")
 config/icon="res://icon.svg"
 run/main_scene.training="res://scenes/training.tscn"
+# Task 6.1: the dedicated_server export feature swaps the boot scene the same
+# way the training export does, so the server binary needs no CLI flag to reach
+# its own entry point.
+run/main_scene.dedicated_server="res://scenes/server_boot.tscn"
 
 [autoload]